The invention relates to an axial piston refrigerant compressor comprising at least one piston-cylinder unit, whose cylinder is closed by a valve plate that has at least one discharge valve with an outlet opening, a projection of the piston extending into the outlet opening, when the piston is near its upper dead center.
From DE 195 15 217 A1 is known a compressor of this kind, in which the piston has an asymmetric projection, which cooperates with the outlet opening of the discharge valve. The outlet opening is adapted to the asymmetric projection of the piston.
From the patent application DK 898/92 is known an axial piston compressor with a conical piston projection, which cooperates with a conical outlet opening of the discharge valve.
From U.S. Pat. No. 5,149,254 is known an axial piston compressor with a recess in the part of the piston front face, which extends from the outlet opening of the discharge valve to the centre of the piston front face. In this recess a piston projection may be provided, which cooperates with the outlet opening.
In these known compressors, the piston projection is supposed to occupy the outlet opening to the largest possible extent in the upper dead centre, to avoid its xe2x80x9cdead volumexe2x80x9d, that is, also to push out the gas contained in the opening and thus to increase the efficiency of the compressor.
The virtual (free) cross-sectional area of the outlet opening is reduced, when the piston approaches its upper centre, so that the flow resistance in the outlet opening increases. The flow conditions in the outlet opening and around the valve closure element may cause the discharge gas to create recirculation zones in part of the outlet opening. The piston projection may aggravate this problem in that the distance between the projection and the outlet opening has already decreased to a flow restriction before the projection has reached the outlet opening. Thus, the free cross-sectional area of the outlet opening may already be substantially restricted, before the discharge valve opens.
The invention is based on the task of providing an axial piston refrigerant compressor of the kind mentioned in the introduction, which has an even higher efficiency.
According to the invention, this task is solved in that the outlet opening, the piston projection, the inside of the valve plate and the front face of the piston delimit a flow channel having a continuous extension of its axial section edges at least over the major part of its circumference. The free cross-sectional area of the flow channel is determined by the smallest cross-sectional area of the outlet opening, at least until the piston, during its pressure stroke, has reached a position, which lies below the upper dead centre by at least the height of the outlet opening. During the further pressure stroke of the piston the relative decrease of the free cross-sectional area of the flow channel is smaller than the relative decrease of the volume of the pressure chamber. In the upper dead centre of the piston, at least 45% of the volume of the outlet opening is occupied by the projection.
This solution gives a flow channel with a minimum flow resistance, a smaller pressure loss in the outlet opening and a smaller xe2x80x9cdead volumexe2x80x9d. The maximum outflow velocity of the gas gets smaller. At the same time, a noise reduction is obtained. On the whole, the improved efficiency of the compressor is improved.
Preferably, the cross-sectional area of the outlet opening decreases in the direction of the outside of the valve plate. It is also preferable that the cross-sectional area of the projection decreases towards its free end and that the cross-sectional areas of the outlet opening and the projection change in the axial direction in such a way that during the piston movement the free cross-sectional area of the flow channel changes relatively less than the volume remaining in the cylinder. Thus, the flow resistance of the flow channel remains at a low level, while during the pressure stroke of the piston the flow or the mass flow decreases.
During the pressure stroke of the piston, the flow resistance of the flow channel can be determined by the smallest cross-sectional area of the outlet opening, until the free end of the piston projection is aligned with the inside of the valve plate. This gives an optimum gas discharge while the mass flow through the outlet opening is at its maximum.
In particular, during the pressure stroke of the piston, the flow resistance of the flow channel can be determined by the smallest cross-sectional area of the outlet opening, until 50% of the height of the piston projection has penetrated into the outlet opening. This gives an optimum gas discharge, until the piston speed has decreased substantially and the gas flow has decreased.
In an embodiment of the present invention, an axial section through the outlet opening of the valve plate and the piston projection has curved section edges. Therefor, the edge of the outlet opening can be steeper than that of the projection.
In particular, the compressor according to the invention can be designed in such a way that the junction surface between the valve plate surface and the outlet opening and the junction surface between the piston front end and the projection have a continuous shape, the junction surface between the outlet opening and the valve seat and the junction surface between the projection and the piston front end being rounded. Thus, gas discharge during draining of the cylinder can take place almost without creating eddies, thus decreasing the flow resistance.
The outlet opening can have an asymmetrical shape. This is advantageous, when the outlet opening is offset in relation to the centre of the cylinder.
Alternatively, the outlet opening can have a symmetric shape. This is advantageous, when the outlet opening is placed close to the center of the cylinder.
Also the piston projection can have an asymmetrical shape. Thus, the projection can be adapted to an asymmetrical outlet opening.
When the piston projection is symmetrical, it can be adapted to a symmetrical outlet opening.
It is also possible to combine a symmetrical piston projection with an asymmetrical valve opening and vice versa.